Grid Standards Explained

For Solis inverters being installed in the United States, a "UL" grid standard is generally appropriate. Examples when searching through the grid standard options are: UL-240V, UL-480V, and UL-600V 


Some areas, like California, would require the California Rule 21 grid standard, which for Solis looks like: R21P3-240, and R21P3-480. There are two versions of each US grid standard, the second version incorporating an "A" for "AFCI" for example: UL-24A, and R21PR-24A.  


For systems within the New England ISO [see map below] jurisdiction, an "ISO" grid standard should be selected. Examples for Solis include: ISONE-24AISONE-480, ISONE-600. 


***The version with "A" in the name must be selected if AFCI is required. Typically, AFCI is required whenever the PV is on a roof, carport, or ground-mount when there is dry grass under the modules.*** 


Common Acronyms: 

Grid standards (codes) in the United States are driven from two different directions. From the top down, the IEEE creates industry standards for interconnection with the electrical grid. The IEEE 1547 standard deals specifically with this very thing. As time goes on, the standard develops as different iterations that typically build on past iterations. From the bottom up, individual ISOs can generate their own set of requirements for DER interconnection with the grid. Common examples of this are California Rule 21, Hawaiian Electric Company Rule 14H, and ISO New England.  

The industry typically rolls the requirements from IEEE 1547 with NEC codes into a UL certification. For the US, this is UL 1741. Click here for more information on UL 1741 and Rule 21.  


ISOs and RTOs have jurisdiction over large geographical areas of the United States with respect to energy generation, transmission, and power grid regulation. These organizations enforce the rules of interconnecting and interoperating with the grid. Grid codes are known as ‘interconnecting guidelines’. They specify requirements for the technical and operational characteristics of power generation plants, such as wind farms, and for different parties involved in the production, transportation and utilization of electric power. Grid codes address all significant concerns associated with the power grid and guarantee its safe operation and performance. Because utility-scale wind farms can replace conventional power plants, wind farms are expected to support power grids and provide ancillary services similar to conventional power plants.  


There are currently seven RTOs or ISOs in the US: 

  • Pennsylvania New Jersey Maryland Interconnection (PJM). PJM coordinates the movement of wholesale electricity in all or parts of Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia. 

  • Midcontinent Independent System Operator (formerly, Midwest ISO) (MISO). MISO assures open access to the transmission facilities and cost-effective delivery of electric power across all or parts of 15 states (North Dakota, South Dakota, Nebraska, Minnesota, Iowa, Wisconsin, Illinois, Indiana, Michigan, Arkansas, Louisiana, Montana, Missouri, Kentucky, and Ohio) and one Canadian province. 

  • Electric Reliability Council of Texas (ERCOT). ERCOT manages the majority of the grid in Texas. Unlike the other RTOs/ISOs, ERCOT operates in only one state and as a result is generally exempt from FERC's jurisdiction, including its regulations regarding rates and transmission. The statute that confers regulatory authority to FERC, the Federal Power Act, applies to the transmission and wholesale of electricity in interstate commerce. 

  • California ISO (CAISO). CAISO provides open and non-discriminatory access to the bulk of California's wholesale transmission grid and some parts of Nevada. Associated agencies: CEC, CPUC, ARB, California State Government. 

  • New York ISO (NYISO). NYISO operates New York State's high voltage transmission network, administers and monitors the wholesale electricity markets in the state, and plans for the state's energy future. While NYISO operates only NY's grid, it is subject to FERC's regulatory authority because the grid is interconnected with other grids in the region. 

  • Southwest Power Pool (SPP). The SSP manages the transmission grid and ensures reliable supplies of power, adequate transmission infrastructure, and competitive wholesale prices of electricity in all or parts of 14 states (Arkansas, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, and Wyoming) 

  • ISO New England (ISO-NE).ISO-NE ensures the day-to-day reliable operation of New England's bulk power generation and transmission system, oversees and ensures the fair administration of the region's wholesale electricity markets, and manages comprehensive, regional planning processes.[Text Wrapping Break][Text Wrapping Break][Text Wrapping Break]Image source:[Text Wrapping Break]These ISOs enforce standard adoption across the country. The standards stem from IEEE 1547 as it evolves over the years. The latest requirements have been laid out in IEEE 1547-2018. The map below shows where the individual states are with enforcing the 2018 version of the standard:[Text Wrapping Break] 

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Victor is the author of this solution article.

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